The invention relates generally to a method and apparatus for preparing synthetic resinous materials for extrusion through a forming die. More particularly, the invention concerns a method and apparatus for preparing a radially stratified annular flow of dissimilar synthetic resinous materials for extrusion through a conventional forming die.
Extrusion of synthetic resinous material through a forming die to produce a self-supporting tube of indefinite length is well-known. However, extrusion of a tubular member of indefinite length having a radially stratified wall with inner and outer skins of one synthetic resinous material and a core of a second synthetic resinous material has presented problems to those skilled in the art.
A typical solution is the use of a multimanifold forming die having appropriate internal channels to distribute the resinous materials prior to extrusion through an orifice of the die to form a tube. However, a multimanifold forming die must be provided for each tube size to be extruded, and in addition, a conventional die must also be available to extrude a tube in the desired size from a single material. Accordingly, the capital investment required for both single layer wall and multiple layer wall tubing production in a range of sizes substantially exceeds the investment required for either single layer wall or multiple layer wall production alone.
An approach to reducing the capital expenditure required has been to use a feedblock in multiple layer wall production that supplies an annular flow of radially stratified material to a conventional forming die. In this manner, the conventional forming die can be used for both single layer and multiple layer tubing.
One coextrusion feedblock is known but it has a configuration that is ill-suited for application to a variety of tubing sizes since scaling introduces substantial circumferential and radial maldistributions in the resulting annular flows. The maldistributions result, in part, from the confluence of decelerating flows and from distribution channels having inherent maldistributions.
The significance of circumferential and radial maldistributions is particularly exacerbated when dealing with self-supporting tubular members having skins comprising only a small portion of the total radial wall thickness. With small thickness strata circumferential maldistribution causes non-uniform wall thickness; radial maldistribution also contributes to non-uniform wall thickness.
The highly complex viscous properties of the synthetic resinous materials of commercial interest is another problem which has detracted from prior attempts to make an effective coextrusion feeding system. The highly viscous nature contributes to maldistribution of one layer with respect to a second layer where one flow is distributed circumferentially about the other. Where a viscous flow having a non-uniform distribution undergoes deceleration, an existing maldistribution is exaggerated.
Attempts to ameliorate such a maldistribution include adjustment of the slot through which a layer is distributed, but, such a design method limits the apparatus to a particular mass flow rate and is, therefore, not acceptable for commercial applications in which different materials and flow rates are necessary.
An example of a multimanifold die for forming multiwalled plastic film is found in U.S. Pat. No. 3,223,761, which issued Dec. 14, 1965. This die, however, forms all layers of the wall simultaneously at the end of a distribution tube. A distribution of this type decelerates the layers at the point of confluence as well as upstream thereof so that maldistributions are exaggerated. Moreover, radial maldistributions cannot be avoided as there is no means to adjust flow conditions of the layers at the point of confluence. The apparatus if further deficient in that the long axial distance between layer formation and development of annular flow allows small external temperature gradients to cause a phenomenon known as thermal drift in which dissimilar synthetic materials move through the apparatus in a non-axial fashion resulting in non-uniform layers.
Accordingly, the need continues to exist for truly effective feedblock which can distribute strata of a second synthetic resinous material on a first synthetic resinous material core for subsequent development into a self-sustaining tubular structure.